I have the following JavaScript code:

var counter = 0;

function printCounter(){

   console.log("counter=" + ++counter);

   setTimeout(printCounter, 1000);

}

printCounter();

I expect that it should print this output:

counter=1

counter=2

counter=3

...

But instead it prints following:

counter=1

undefined  // <-- Notice this "undefined"

counter=2

counter=3

...

Why does it print "undefined" after the first iteration?

Important: I see such behavior only when the code executed in the JavaScript console. If it's the part of a page, it works fine.